Heretofore, as one example of techniques in this field, Patent Documents 1 to 4 listed below disclose that a ring gear is press-fitted into an outer circumference of a differential case for fastening the ring gear to the differential case. Especially, Patent Document 1 discloses the technique that a ring gear is press-fitted in a flange of a differential case, and after that, inner peripheral edges at opposite ends of the ring gear are swaged or deformed by swaging or deforming parts at both ends of the flange so as to fix the ring gear. Then, predetermined components as well as the ring gear are assembled to the case, and a differential sub-assembly used for a power transmission mechanism of a vehicle is obtained.